This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-339332 filed on Nov. 7, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method of correcting patterns to be formed on a photomask used for photolithography in semiconductor device manufacturing, a photomask with corrected patterns, and a computer-readable storage medium storing a mask pattern correcting program.
2. Description of the Related Art
To provide finer semiconductor devices, finer patterns must be formed on a wafer by photolithography employing a photomask and an optical projection system.
If the optical projection system has sufficient resolution for original patterns designed for a semiconductor device (such original patterns as designed are hereinafter referred to as the xe2x80x9cdesign patternsxe2x80x9d), patterns formed on a photomask (sometimes simply referred to as a xe2x80x9cmaskxe2x80x9d) may exactly be faithful to the design patterns, to form the design patterns on a wafer by photolithography using the photomask. If the design patterns are too fine for the resolution of the optical system and if they are exactly formed on a photomask, the photomask will form thinned or thickened patterns on a wafer due to an optical proximity effect (OPE). In this case, the patterns on the wafer are unfaithful to the design patterns.
To cope with this problem, an optical proximity correction (OPC) has been carried out on mask patterns.
FIG. 1 shows an exposure shot area A on a photomask. The exposure shot area is an area of a photomask that is exposed to a shot of light during a photolithography process to print patterns on a wafer through the photomask. To carry out the optical proximity correction on a line pattern in the exposure shot area A, rule-based correction or simulation-based correction is employed.
FIGS. 2A to 2C explain a rule-based correction technique. In FIG. 2A, a correction target pattern 100 is surrounded by peripheral patterns with certain distances between them. The rule-based correction prepares, in advance, a correction table showing relationships between pattern-to-pattern distances and correction quantities. This correction table is arranged based on tests that measure differences between design patterns and corresponding actual patterns formed on a wafer.
In FIG. 2B, a design pattern 101 depicted with a dotted line has a distance S from an adjacent pattern. If the design pattern 101 is faithfully formed on a mask, it will project a thinned pattern 102 on a wafer due to the optical proximity effect when the wafer is exposed to light through the mask. The rule-based correction retrieves a correction quantity B from the above-mentioned correction table according to the pattern-to-pattern distance S and adds the retrieved correction quantity B to the design pattern 101, to form a corrected pattern 103 on a photomask as shown in FIG. 2C.
FIGS. 3A to 3C explain a simulation-based correction technique. FIG. 3A shows a correction target pattern 110. FIG. 3B shows a design pattern 111 corresponding to the pattern 110. A simulation range Sa of several micrometers is defined around the design pattern 111. In the simulation range Sa, a simulation is carried out to simulate a pattern 112 as a pattern to be actually formed on a wafer by photolithography. The difference between the design pattern 111 and the simulated pattern 112 is a correction quantity. Based on the correction quantity, a corrected pattern 113 is formed on a photomask as shown in FIG. 3C.
The related arts mentioned above employ a correction target pattern and a small peripheral area of several micrometers around the correction target pattern to cope with the optical proximity effect.
According to recent studies, circuit patterns formed by photolithography including exposing, developing, and etching processes are thinned or thickened not only by conditions in a peripheral area of several micrometers square but also by conditions in a wider peripheral area of several hundreds to thousands of micrometers square. As a result, the related arts are improper to correctly form patterns as designed on a wafer.
A first aspect of the present invention provides a method of correcting mask patterns. The method obtains pattern correction quantities according to differences between mask patterns and patterns actually formed on a wafer by photolithography using the mask patterns, prepares pattern density-based correction data from the obtained pattern correction quantities, obtains design patterns for a correction target area defined on a mask, calculates a density of the design patterns in the correction target area, retrieves correction data corresponding to the calculated density from the pattern density-based correction data, and corrects the design patterns for the correction target area according to the retrieved correction data.
A second aspect of the present invention provides a method of correcting mask patterns. The method prepares pattern density-based correction data like the first aspect, obtains design patterns for an exposure shot area defined on a mask, calculates a density of design patterns for a discrete circuit block requiring a highest pattern accuracy in the exposure shot area, retrieves correction data corresponding to the calculated density from the pattern density-based correction data, and corrects the design patterns for the correction target area according to the retrieved correction data.
A third aspect of the present invention provides a method of correcting mask patterns. The method prepares pattern density-based correction data like the first aspect and obtains design patterns for a correction target area defined on a mask and design patterns for a peripheral area defined around the correction target area. The xe2x80x9cperipheral areaxe2x80x9dis an area whose pattern density affects pattern sizes in the correction target area. The method calculates a density of the design patterns for the correction target area and a density of the design patterns for the peripheral area, calculates an effective pattern density for the correction target area according to the calculated two densities, retrieves correction data corresponding to the effective pattern density from the pattern density-based correction data, and corrects the design patterns for the correction target area according to the retrieved correction data.
A fourth aspect of the present invention produces a photomask according to the method of the first aspect.
A fifth aspect of the present invention produces a photomask according to the method of the first aspect.
A sixth aspect of the present invention produces a photomask according to the method of the third aspect.
A seventh aspect of the present invention provides a computer-readable memory storing a program to execute the method of the first aspect. The program includes reading design patterns to be contained in a correction target area defined on a mask, calculating a density of the design patterns in the correction target area, retrieving correction data corresponding to the calculated density from pattern density-based correction data prepared in advance according to differences between mask patterns and patterns actually formed on a wafer by photolithography using the mask patterns, and correcting the design patterns to be contained in the correction target area according to the retrieved correction data.
An eighth aspect of the present invention provides a computer-readable memory storing a program to execute the method of the second aspect. The program includes reading design patterns for a correction target area defined on a mask, calculating a density of design pattern for a circuit block requiring a highest pattern accuracy in the correction target area, retrieving correction data corresponding to the calculated density from pattern density-based connection data prepared in advance according to differences between mask patterns and patterns actually formed on a wafer by photolithography using the mask patterns, and correcting the design patterns for the correction target area according to the retrieved correction data.
A ninth aspect of the present invention provides a computer-readable memory storing a program to execute the method of the third aspect. The program includes obtaining design patterns for a correction target area defined on a mask and design patterns for a peripheral area defined around the correction target area, calculating a density of the design patterns for the correction target area and a density of the design patterns for the peripheral area, calculating an effective pattern density for the correction target area according to the calculated two densities, retrieving correction data corresponding to the effective pattern density from pattern density-based correction data prepared in advance according to differences between mask patterns and patterns actually formed on a wafer by photolithography using the mask patterns, and correcting the design patterns for the correction target area according to the retrieved correction data.